The rotors and housings of turbo machines, such as steam turbines, compressors, gas turbines and the like are generally designed on the basis of strength and/or weight, and housings of relatively small mass are opposite rotors of relatively large mass. The following characteristics result from this:
An optimal clearance of, for instance, a few hundredths of a millimeter can be provided for the radial rotor and stator guide vanes or blades only for a specific load condition. Other load conditions must be carried out with radial slot sizes based on the above optimizing design. However, the slot size varies not only for load conditions, but also between cold and hot conditions and in the thermal transition behavior of the rotor and stator between different loading conditions;
In the event of excessive differences in the thermal transition behavior it may be necessary to make the "optimum" radial slot larger in order to avoid, in certain transient conditions, a radial scraping of the rotor and stator guide vanes with their respective opposed structure;
The above is also important in the construction and sizing of the slots of radial seals.
As a result of this conventional construction, there are a number of disadvantages as noted hereafter:
Loss of power and efficiency, or increased consumption of fuel for load conditions which differ from that for which the slot sizes were optionally designed;
Loss of power and efficiency, or increased consumption of fuel during transition states or non-uniform operating conditions;
Susceptibility to compressor pumping, particularly during starting and acceleration.
In order to provide a margin of safety in light of the varying conditions, it is known from U.S. Pat. No. 4,329,114 to provide a radial slot control device, which is adjustable as a function of engine output parameters, for compressors of gas turbine engines In the patent there is disclosed a construction in which air taken from a relatively "cold" region of the compressor is selectively diverted, by a flap control valve on the outer casing, to flow either in entirety or in part along the outside of the corresponding inner stator vane support structure, and therefore over an axial length from the region of air removal approximately to the last compressor stage.
In another system for controlling the clearance between vanes or blades and opposite structure for compressors of gas turbine engines, U.S. Pat. No. 4,338,061 shows a system operating predominantly electronically and including a mechanical control valve by which relatively cold air is bled for turbine cooling or control of internal leakage. The cold air is bled from a comparatively early compressor stage, for instance, from the fifth compressor stage as a by-pass flow mainly for control of the size of the radial slot due to blade clearance. In this regard, the bleed point communicates both with an outer first flow path extending along the compressor housing up to the last compressor stage and with a second flow path extending parallel to the latter. The mechanical control valve controls a variable passage of air through both flow paths and thus provides cooling of variable intensity of the outer housing. The optimal position of the control valve for the specific operating condition is representative of the size of the radial slot actually required at the time, as calculated by a computer system, using pertinent engine parameters based on the difference between the actual temperature and the desired temperature of the housing as predetermined for the condition of particular operation.
The following disadvantages result from the systems of the two U.S. Patents described above:
An expensive electronic construction is required;
Expensive air guides and control means are required;
A comparatively large increase in weight is obtained by the added structure;
A larger engine diameter is required, particularly in the region of the compressor;
It is necessary to tap off comparatively large amounts of compressed air which compromises the compression; and
As a whole, considerable susceptibility to turbulence is produced.
From the journal "INTERAVIA," 2 (February), 1983, page 102, middle column, last paragraph, there is known, by itself, a so-called "active" slot control for the compressor of a gas turbine engine by introduction of hot air into the corresponding compressor rotor.
U.S. patent application Ser. No. 758,049, filed: 7/23/85 provides an apparatus for optimizing the size of the slot or clearance of the rotor and stator vanes in axial flow compressors of gas turbine power plants to conduct a leakage air stream, discharged from the main compressor seal in the region of the end of the compressor in a direction opposite the main direction of air flow in the compressor towards tee last and the penultimate rotor disks whereafter the air is discharged for further use, for example, for cooling purposes.
This has the disadvantage that the amount of removal air is dependent on the slot behavior of the seal which varies in accordance with load. Such seals are generally constructed as labyrinth seals. Therefore, the quantity of leakage air is subject to relatively great variation and the desired continuous effectiveness of heating of the rotor parts cannot be obtained. Particularly during transient or non-uniform operation and especially for relatively small sizes of the slot of the main seal, the required quantity of removal air can no longer be obtained in sufficient amount.
Another significant disadvantage of the above-discussed construction is that the leakage air flowing from the main seal and used for the local heating or venting of structural parts has a relatively high temperature. In this respect, the compressed air at relatively high temperature which flows from the last compressor stage through the main seal undergoes an additional increase in temperature as a result of additional air friction between rotating and stationary parts of the main seal. There may also be other increases i the air temperature due to air friction between a stationary guide wall, such as the outer wall of a combustion chamber and a rotating support disk which supports the rotating part of the seal. Consequently, at the least the last three rotor disks are contacted by substantially the same relatively hot air in the region of their greatest mass close to the axis of rotation of the rotor. Hence, a locally precise thermal control of the compressor rotor for optimization of the radial slots proportional to the continuous increase in temperature of the compressed air flow in the compressor is not assured.
Further U.S. Pat. Nos. 3,742,706 (Klompas) and 3,844,110 (Widlansky et al) provide--among others--a combined compressor/turbine/cooling arrangement for a gas turbine engine wherein relatively cool air is taped off at an upstream stage from the compressor and is conducted at a greatly reduced temperature against adjacent disks of the compressor rotor. In other words there is no possibility to heat up the rotor disks of the compressor to temperature levels which follow the continuous increase in temperature of the compressed air flow in the compressor.